Passwords are always kept secure, whether it is in your device or on web.

You might have saved password of a WiFi network in your phone and have forgotten or your friend have saved a WiFi password in your device. 
Don't worry! There are some ways which will help you to reveal it.

Method 1:

If you have a rooted phone, then you can download a simple app to view it.

Download link: WiFi Pass viewer

Method 2:

This one is a bit lengthy but perfect one.

You can find the password also by using some file manager like Root Explorer, ES file manager.

You only have to follow these paths:

1. Open the file manager.  
   
2. Goto root>data>misc>wifi .(In ES file explorer, root folder is named '/device' but it is named as 'system root' in FX)
3. There you'll find some files.
4. Open wpa_supplicant.config

You will find the WiFi ssid and their password saved with other details.

Method 2 can also work on some unrooted phones.